This invention relates to a remote lens focusing control system for an airborne reconnaissance camera and, more particularly for a digital lens focusing control system.
Navy patrol aircraft were originally outfitted with a six inch focal length camera. The camera was suited for the requirements set by the Navy at that time, however, as requirements changed the six inch focal length camera was replaced with an 18 inch focal length camera. This camera provided the long range patrol aircraft with various standoff photographic ranges. However, the lens was designed with a manual focus control and could not be focused during a flight but had to be focused to a desired range prior to flight. After some flights, the necessity for a remote refocusing control system became apparent.
Certain of the focusing systems in the prior art adjust the focus of the lens to an object distance corresponding to the distance of the camera to a remote subject in a field of view of that lens. Specifically, these focusing systems utilize image correlation in which optical systems are scanned by a sensor and the images therefrom then correlated electrically utilizing analog techniques. The output from an analog system normally controls a motor which drives a focusing cell of an objective lens so as to yield focusing when the two images are coincident. Essentially the focusing systems in the art for still and movie cameras are accomplished by controlling through analog means the position of an objective lens system in accordance with a photoelectrically sensed distance from the camera to an object which is to be photographed. An analog system has the inherent disadvantages of being susceptible to heat, noise, shock and vibration. These negative features are particularly bothersome on an aircraft where there are a multiplicity of electrical noises, a potential for high shock forces and abnormal ambient vibration levels.